This invention relates to inertially augmented landing systems and more particularly methods and apparatus for overcoming delays in detection of GLS input signal errors essential to safe guidance in landing and rollout of an aircraft.
Since 1993, the industry has been working to develop automatic landing capability using differential GPS. This capability is known as the GNSS Landing System GLS). GLS developments to support CAT 1 operations are nearly complete. The industry is now working on standards and performance requirements for GLS to support CAT II/III operations. A key issue associated with GLS CAT II/III operations is the expected failure modes and effects of the GLS guidance system. It is anticipated that the most common failure mode for GLS will be a total loss of the signal for hundreds of seconds. U.S. Pat. No. 6,178,363B1 shows a GPS/Inertial filtering scheme to enable the airplane to continue to land and roll out after a total loss of GLS guidance below the alert height.
Key to this concept is the ability of the GLS groundstation to provide the aircraft systems with the information required to determine with certainty when the GLS guidance signals are unusable. The airborne multi-mode receiver (MMR) must respond rapidly to switch away from the faulty GLS signals to updated inertial guidance in order to prevent the inertial signals from becoming corrupted by the errors in the GLS signals. Unfortunately, the GLS groundstation cannot communicate the status of the guidance signals instantaneously, and therefore the likelihood of corruption exists. Accordingly, the present method and apparatus as hereinafter described address this problem.
During a failure condition, it is possible for a differential GPS ground station to provide corrupted data for up to 3 seconds before raising an alarm. Furthermore, the airplane is allowed to continue to use the last data provided by the ground station for up to 3.5 seconds after the ground station stops transmitting data. Consequently, there could be a 3-6 second delay between GPS signal corruption and detection of the corruption by the airborne receiver. The present invention provides a means for correcting the integrated GPS/INS solution and protecting the airplane landing performance from any effects due to this potential for data corruption. A skipping filter in combination with a coasting filter shown in U.S. Pat. No. 6,178,363B1 enables recovery from up to 6 seconds of corrupted GPS signal, thereby avoiding subsequent missguidance from the anomalous GPS signal.